In color electrophotography, a full color image is built up by sequentially transferring individual color separation toner images in registration onto a receiver and fusing the toner and receiver. A clear toner can also be provided over the color separation toner images. Such a clear toner protects the color separation toner images from damage due to environmental conditions or from incidental contact.
A clear toner can also improve the gloss of the full color image. Gloss is an optical property that represents the extent to which a surface such as an exterior surface of a fused toner image reflects light at an angle that mirrors an angle of incidence of that light. Several factors can influence the gloss of a toner image fused to a receiver. The primary factors include the general uniformity of the refractive index of the toner used to form the exterior surface of the fused toner image, the flatness of the exterior surface of the fused toner image, and in certain circumstances, the gloss of the receiver.
It will be appreciated that a full color toner image can have an exterior surface that includes toner from any of the color separation toner toners as may be necessary to provide the desired combination of colors and the index of refraction of the toner that is present at an upper layer of the full color toner image can vary with the index of refraction of the color separation toner that is last applied at each layer of the toner stack. Light that strikes the exterior surface at an angle of incidence can be reflected at different angles because of such differences in the index of refraction. Accordingly, a more uniform index of refraction can be provided at an exterior surface of a fused color toner image by providing a common clear toner over the color separation toners.
It is known in the art to apply such a clear layer to color separation images using a clear coating apparatus that applies, for example, a generally uniform coating of a clear material and that fixes the clear material to the toner image by exposing this material to ultraviolet light. For example, Schulze-Hagenest, et al., disclose UV-curable toners for use to form durable prints on paper and cardboard substrates in UV-cured Toners for Printing and Coating on Paper-like Substrates, 13th International Conference on Digital Printing Technologies (Imaging Science and Technology, 1997) pp. 168-172. Also described is apparatus for the UV curing (crosslinking) of such UV-curable toners at elevated temperatures, i.e., above the glass transition temperature (T.sub.g) of the toner. A radiant fusing step, using IR radiation to heat the toner, is followed by a separate UV curing step in which the toner is in a molten or quasi-molten state. The IR pre-fusing provides a smooth film, while the subsequent UV curing reaction is very rapid. UV-crosslinkable toner formulations are disclosed in U.S. Pat. No. 6,608,987 issued to Bartscher, et al. and in U.S. Pat. No. 5,905,012 issued to De Meutter, et al.
In another example, U.S. Pat. No. 5,926,679, issued to May, et al., discloses that a clear (non-marking) toner layer can be laid down on a photoconductive member (e.g., imaging cylinder) prior to forming a marking particle toner image thereon, and that a clear toner layer can be laid down as a last layer on top of a marking particle toner image prior to transfer of the image to an intermediate transfer member (e.g., blanket cylinder). It is also disclosed that a clear toner layer can be laid down on a blanket cylinder prior to transferring a marking particle toner image from a photoconductive member. In one aspect of this patent, a non-imagewise clear toner layer is bias-developed on to an intermediate transfer member using a uniform charger and a non-marking toner development station. A first monocolor toner image corresponding to one of the marking toners is transferred to the ITM (on top of the clear toner) from a primary imaging member which may be a roller or a web but is preferably a roller. Subsequently, a second monocolor toner image corresponding to another of the marking toners is transferred to the ITM (on top of and in registration with the first toner image) and so forth until a completed multicolor image stack has been transferred on top of the clear toner on the ITM. The ITM is then positioned at a sintering exposure station; where a sintering radiation is turned on to sinter the toner image for a predetermined length of time.
The clear toner that is applied to the color separation toner images in accordance with such methods can provide the protective function and can also create a generally uniform index of refraction at the exterior surface of a fused toner image formed on the receiver after fusing to provide improved gloss performance.
However, differences in the amount of color separation toner applied to form different colors form what are known as toner stacks and can cause different the toner stacks to have a different toner stack heights. The difference between toner stack heights can cause relief differentials to exist in the exterior surface of the fused toner image. The relief differentials disrupt the flatness of the exterior surface of such a color toner image. These relief differentials cause light to reflect along different paths and this, in turn, reduces the apparent gloss of the fused toner image.
This effect can be illustrated by reference to FIGS. 1 and 2. FIG. 1 depicts an exemplary section of a receiver member 2 having a plurality of color toner stacks 4A-4N. As can be seen from FIG. 1, color toner stacks 4A-4N provide a range of color toner stack heights before fusing, with the toner stack heights varying based upon the total amount of color toner in each toner stack. As is also seen in FIG. 1, a uniform layer of clear toner uniformly increases the toner stack heights leaving the magnitude of any toner stack height differences unchanged but at a higher level relative to receiver 2.
FIG. 2 shows the section of FIG. 1 after fusing. As is shown in FIG. 2, the pressure and heat applied during a typical fusing process tends to cause the color toner stacks to be pressed together to form a toner mass 6 having an exterior surface 8. As is also illustrated in FIG. 2, exterior surface 8 has a relief pattern with peaks that generally correspond to locations on the receiver member 2 on which higher toner stacks 4A-4N are formed and valleys that generally correspond to locations on the receiver member 2 having comparatively lower toner stacks.
For example, a peak area 10 on surface 8 that corresponds to high density color image elements is shown in FIG. 1 as being formed at areas of the toner image formed by toner having comparatively higher toner stack heights e.g. toner stack 4D and a valley area 12 that corresponds to lower density color image elements shown in FIG. 1 as having a lower toner stack height e.g. toner stack 4E in FIG. 1. Such relief differentials reflect incident light from a common source (not shown) in different directions thereby creating a reduction in gloss. For example, as is shown in FIG. 2, parallel rays of light 14A, 14B and 14C strike different portions of fused toner 8, and are at least in part reflected by exterior surface 8 as reflected rays of light 16A, 16B and 16C that travel in different directions. Accordingly, only a portion of the parallel rays 14A, 14B and 14C can be seen by an observer or detector at a position 18 that mirrors the angle of incidence of the parallel rays 14A, 14B, and 14C on surface 10. This reduces the overall apparent gloss level of the toner image formed on receiver member 2.
It will be appreciated from this that the application of a clear toner in amounts that vary inversely with an amount of color toner in a toner stack can reduce these relief differentials and improve gloss. Accordingly, there have been various attempts to use imagewise application of a clear toner to help form a fused toner image having reduced relief differentials. Often this is done by determining a pattern of clear toner that is calculated to provide reduced relief differentials when applied to the toner stacks formed by the color separation toner images that will be applied to a receiver. This pattern is then converted into the form of image data that can be printed by a printing module to provide a toner image that has reduced relief differentials after fusing.
For example, U.S. Pat. No. 5,234,783, issued on Aug. 10, 1993, in the name of Yee S. Ng, et al., describes a process where a gloss of a printed image is improved by applying gloss improving clear toner image to the color toner stacks forming the image. The gloss producing clear toner image provides clear toner in amounts that vary inversely according to the amounts of toner provided by the color separation images providing ultimately an even height toner image. Similarly, U.S. Pat. No. 7,016,621, issued on Mar. 21, 2006 in the name of Yee S. Ng, describes the formation of a toner image wherein back-transfer artifacts are reduced or eliminated without the need or expense of providing uniform coverage of clear toner to the print wherein a five color tandem printer is used to print fewer than five colors. In this patent, the first four printing stations are used to print a color toner image having a range of stack heights and a fifth station is used to deposit a clear toner image having less clear toner in areas of the color separation toner images having more color separation toner and more clear toner in areas of the color toner image having lower amounts of color separation toner.
Such relief reducing applications of toner are known as inverse mask toner images. The use of inverse mask toner images provides high gloss outcomes by helping to cause exterior surface 8 of a fused color toner image to have a consistent index of refraction and reduced relief differentials. Such inverse mask methods can require the use of a printing module to selectively apply clear toner to specific color toner stacks, requires calculation to determine which toner stack are to receive the amounts of clear toner applied according to the inverse mask, requires that the clear toner is carefully written and transferred in register to the underlying color toner stacks. These steps can require precise calculation, electrical and mechanical control.
It will also be understood that in an electrophotographic printer, a development process is used to deposit toner onto a surface. In this process, a development station supplying charged toner is provided in close proximity to an engine pixel location on a primary imaging member. The difference of potential is established across the toner and the picture element location. Toner deposits onto to the engine pixel location according to the difference of potential therebetween. However, the difference of potential decreases as charged toner transfers to the picture element location. Accordingly, while the net difference of potential at the start of a development step can be high, this net difference of potential decreases as development progresses, slowing the development process and effectively limiting the overall amount of toner developed onto picture element locations of the primary imaging member.
Development efficiency can be characterized as a ratio of a difference of potential between a development station and the engine pixel location during development and a difference of potential between development station and the toned pixel. Development efficiency limitations can be particularly noticeable when the difference of potential between a development station and the charge at the engine pixel location being developed is relatively low or where development efficiency varies during development of an image. Further, in toner images that use multiple layers of color toner, there can be significant differences in the development efficiencies for each layer of toner applied. These development efficiency differences can exacerbate relief differences that already exist between large toner piles formed in high difference of potential areas and comparatively low difference of potential areas that will have low toner stack heights.
Various schemes are known in the art to provide improved development efficiency. These typically seek to improve the development efficiency of a single toner by positioning multiple development stations along a primary imaging member in order to present the same toner to the same portions of a primary imaging member multiple times effectively increasing the amount of time during which development can occur and allowing full development at lower potentials. The overall development efficiencies of each color separation will be closer to a desired development efficiency. Examples of such methods include U.S. Pat. Nos. 3,724,422 issued to Latone et al.; 3,927,641 issued to Handa, 4,041,903 issued to Katakura et al. Such approaches can improve toner development efficiency but require additional structure to enable the formation of an inverse mask.
What are needed therefore are new methods and apparatuses for applying an inverse masking toner to toner stacks formed from one or more color separation toners forming a toner image in amounts that vary inversely with the amount of color separation toner in the toner stacks to form an exterior surface of the fused toner image that has a more uniform index of refraction and reduced relief differentials. Another need in the art is for methods and apparatuses to be provided that allow application of inverse masking toner to compensate for development efficiency limitations. Still another need in the art is for methods and apparatuses to be provided that allow the formation of such an inverse mask toner without requiring calculation of second toner amounts based on analysis of color separation data, without requiring an image printing module to selectively position the inverse masking toner relative to the toner stacks or to adjustably control the amount of inverse mask toner applied to particular toner stacks.
What are needed therefore are new methods and apparatuses for applying an inverse masking toner to toner stacks formed from one or more color separation toners forming a toner image in amounts that vary inversely with the amount of color separation toner in the toner stacks to form an exterior surface of the fused toner image that has a more uniform index of refraction and reduced relief differentials.
Still another need in the art is for methods to be provided that allow the application of such a protective and gloss improving toner in specific amounts on specific toner stacks in toned portions of a receiver. This requires precise registration with the toner stacks formed in the color toner image. Even minor mis-registration can yield highly unpredictable results that can increase relief differentials and decrease rather than increase gloss.
Yet another need in the art is for methods and apparatuses to be provided that allow application of inverse masking toner to compensate toner stack height variations without requiring calculation of second toner amounts based on analysis of color separation data, without requiring an image printing module to selectively position the inverse masking toner relative to the toner stacks or to adjustably control the amount of inverse mask toner applied to particular toner stacks.